Static Ward-Leonard four quadrant control

The static Ward-Leonard drive replaces the M–G set of the conventional Ward-Leonard system by a thyristor phase-controlled converter. For four-quadrant control, a dual converter (two fully controlled converters connected back to back) feeds the armature of a separately excited dc motor, whose field is kept constant.

Scheme

          +--[Converter P (+I)]--+
 ac ------|                      |---[ M ]  field const
          +--[Converter N (-I)]--+
   P: alpha_P,  N: alpha_N = 180 - alpha_P

• Converter P carries positive armature current; converter N carries negative current.
• Each converter can give positive or negative average voltage ($V = V_{do}\cos\alpha$).

To brake from forward motoring, P is blocked, N is fired at $\alpha_N > 90^\circ$ so that its voltage is just below $E_b$; current reverses and energy returns to the supply.

Non-circulating current scheme

• Only one converter conducts at a time; the other is blocked (no firing pulses).
• On reversal, current in the working converter is brought to zero, a dead time (about 2–10 ms) is allowed for thyristors to recover, then the other converter is fired.
• No inter-group reactor needed; smaller converters, higher efficiency.
• Slower response and possible discontinuous current near zero.

Circulating current scheme

• Both converters are fired continuously with $\alpha_P + \alpha_N = 180^\circ$, so their average voltages are equal and opposite in sign around the loop.
• Instantaneous voltages differ, so a reactor is connected between them to limit the circulating current.
• Current can pass smoothly from one converter to the other without dead time: fast, smooth reversal and continuous conduction.
• Costlier: reactor, converters rated for load plus circulating current, extra losses.

Merits over conventional Ward-Leonard

• No rotating machines: less space, weight, foundation, noise and maintenance.
• Higher efficiency (above 95 %) and fast response (milliseconds).
• Lower cost for most ratings; easy closed-loop control.
• Regeneration still possible.

Drawbacks: harmonics and poor power factor at large $\alpha$ (low speed).